Can i call a phone from my # and have it show coming from another #?

a freind is having a problem with his ex saying he is calling her from his phone. the calls show his # on her phone but the bill shows no calls to her #. she has told the police he is doing it from his computer , that it is easy. how is this possible ? anyone know how this is done ? also how can u send an e-mail showing a different ip address ? need help quick.